In high speed turbomachines, e.g. steam or gas turbines, multiple turbine stages, each comprising a plurality of circumferentially distributed blades forming a turbine wheel, are arranged axially along a rotatable shaft. The turbine wheels rotate in response to the force of a high pressure fluid flowing axially through the machine and impinging on the blades of the wheels. Natural resonant frequencies of the blades may coincide with or be excited by some rotational speeds and integral harmonics thereof. Blade resonances excited at multiples of the shaft rotational frequency may create stresses which break one or more blades and cause extensive damage, thus shutting the machine down and requiring extensive repair.
To avoid resonances, blades in the low pressure sections of steam turbines are tuned to avoid excitation at multiples of operating speed. This tuning is achieved by careful analysis during blade design. Detailed testing is performed prior to operation of a machine to ensure that new blades do not resonate during normal operation. A rotating test of a row of turbine blades comprises excitation of the blades with a fluid jet while measuring the vibratory response of several blades with strain gages to determine the frequencies of resonance, that is the excitation frequencies at which the greatest response occurs. Such a steady fluid jet excites only frequencies which are integral multiples of shaft rotational speed. The shaft speed must be varied in order to vary the excitation frequency. Stringent quality control practices are then followed to assure that the blades are manufactured as designed. These quality control measures depend upon laboratory testing to set manufacturing tolerances and to verify blade tuning. However, because individual evaluation of manufactured blades can be time intensive, it has not been practical to laboratory test all blades under normal rotating conditions in order to confirm proper blade tuning. On the other hand, blade testing in non-operational environments has been an imperfect alternative because it requires correction of test data in order to predict vibratory responses under rotating conditions. These adjustments are necessary because resonant frequencies vary with changes in blade stress resulting from centripetal forces during operation.
It is also desirable to monitor rotating blades during operation in order to identify vibration problems which develop after a turbomachine is put into use. This on-line evaluation is necessary in part because evaluations performed prior to actual use, even rotational tests, do not subject the blades to the same forces, temperatures and pressure conditions which are experienced during field operations.
Continuous monitoring of blade vibrations is also important in order to detect shifts in resonant frequencies which signal structural changes. For example, a propagating crack will cause the resonant frequencies of a blade to decrease. It is desirable to detect these changes before the blade becomes resonant at the shaft rotational speed or a harmonic thereof. Otherwise the vibrating blade may undergo dangerously high stresses. Other factors also cause resonant frequencies of the blades to change with time. For example, corrosion and erosion of airfoil areas may also change resonant frequencies and changes in riveted or welded joints by which some blading assemblies are fastened together and to the turbine shaft may alter resonant frequencies.
The model frequencies of rotating blades also depend on the fit of blades in the rotor attachment grooves which secure the blades in place. The dynamic effect of high speed rotation normally improves the securement of a blade because centripetal forces tighten the connection. This dynamic loading at operating speed is difficult to simulate. The frequency response of a blade is a strong function of operating speed, because centripetal forces both stiffen the blades and enforce their connection to the rotor. The resulting variation of resonant frequency with speed must be determined if stationary test data are used.
Although stationary testing in combination with appropriate correction data can provide meaningful information for new blades, corrosion in the rotor attachment grooves is known to affect blade securement and change both the stiffness of a blade and its damping characteristics. Thus, factory test data do not necessarily correspond to the characteristics of blades found in older machinery. Furthermore, similar adjustments to stationary test data may not result in the correct dynamic characteristics of retrofit blades, again because age effects alter the original tolerances in rotor attachment grooves. These variable physical changes cannot be fully accounted for without direct measurements.
Although previous methods of performing evaluations have successfully eliminated some serious vibration problems, it is desirable to perform reliable and comprehensive monitoring in order to further avoid the above described problems. In the past there has been a very limited capability for monitoring on-line blade vibrations, but with recent advances in blade vibration monitoring, fast, long-lived and cost-effective monitoring systems are now able to provide and continuously update blade vibration information for entire rows of blades in turbomachines. An exemplary system is disclosed in U.S. Pat. No. 4,573,358 assigned to the assignee of the present invention.
With the advent of improved systems for blade vibration monitoring it is desirable to periodically measure blade resonances in operating equipment in order to detect structural changes. In the past this has not been possible because on-line monitoring of blade vibrations has been limited to passive evaluations, i.e., to the detection of naturally occurring resonances which correspond to the frequency of shaft rotation or harmonics thereof. A disadvantage of passive evaluation is, of course, that the shifts in resonant frequencies which signal structural defects may not be discoverable before extensive damage results. One reason that shifts in blade resonances are not monitored on-line is that there has not been available a method for exciting the blades of a turbomachine at variable frequencies during normal operation. Clearly such a method in combination with a suitable blade vibration monitoring system would provide reliable and accurate data acquired under the most realistic conditions.